If you measure the distance around a circle and divide it by the distance across the circle through the center, you will always come close to a particular value, depending upon the accuracy of your measurement. This value is approximately 3.141592… We use the Greek letter PI (pronounced Pi) to represent this value. The number PI goes on forever. However, using computers, PI has been calculated to over 1 trillion digits past the decimal point.
The distance around a circle is called the circumference. The distance across a circle through the center is called the diameter. PI is the ratio of the circumference of a circle to the diameter. Thus, for any circle, if you divide the circumference by the diameter, you get a value close to PI. This relationship is expressed in the following formula: C / D = PI
where C is circumference and D is diameter. You can test this formula at home with a round dinner plate. If you measure the circumference and the diameter of the plate and then divide C by D, your quotient should come close to PI. Another way to write this formula is: C= PI x D where x means multiply. This second formula is commonly used in problems where the diameter is given and the circumference is not known.